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Image of the Week

Sky coverage of Gaia during Commissioning

This "skymap" figure illustrates the areas of the sky that Gaia has already observed during its five months of on-going commissioning.

The axes are Equatorial coordinates; the colour scale indicates the measurement density (as observations per square degree). Such density depends both on the sky itself and on the "scanning law" followed by Gaia.

As can be seen, there are areas where the density is significantly higher than in others. This is due to the special scanning laws in which Gaia is being operated during commissioning. Specifically, there are two small areas with a very high density: the "ecliptic poles", that is, the points on the celestial sphere crossed by an imaginary line perpendicular to the ecliptic plane - where the Earth travels on its orbit around the Sun.

This feature is intentional: The commissioning phase was designed to run mostly on the so-called "ecliptic poles scanning law" (or EPSL), in which the spin axis of Gaia is completely fixed with respect to the Gaia-Sun axis (not doing any precession motion around it). Using the EPSL, the ecliptic poles have been observed every day, allowing DPAC scientists to use the stars in such poles as references and perform essential diagnostics on the satellite, its instruments and the on-ground data processing systems. The EPSL mode can be either "trailing" or "leading", that is, with the Gaia spin axis at -45 or +45 degrees with respect to the Gaia-Sun axis on the ecliptic plane.

Different manoeuvres and scanning modes of the satellite have also been tested during these months. (For example, setting the spin axis to zero degrees with respect to the Gaia-Sun axis or "sun aspect angle"). Also, and specially, the "nominal scanning law" (NSL), which is the mode in which Gaia will operate during the nominal mission, has been tested. This has been done twice so far - including the very recent "Galactic Plane scan", in which a very high number of stars was observed, almost following the Galactic Plane to stress-test all the systems.

This movie illustrates all these manoeuvres. It starts with the EPSL-trailing mode, followed by the EPSL-leading mode (including a short period with a sun aspect angle of zero degrees). It then shows the NSL mode which had been tested a few days after the EPSL modes. Another round of EPSL-leading is then followed by the NSL scanning close to the Galactic Plane.

These "skymap" figures were automatically determined by the Initial Data Treatment from the data processed by this system every day, using the star measurements and the attitude information provided by the satellite. The composed image and movie have been prepared by the University of Barcelona team.